This invention relates generally to gas generant compositions, such as those used to inflate automotive inflatable restraint airbag cushions and, more particularly, to burn rate-enhanced, high gas yield non-azide gas generant compositions.
The burning rate for a gas generant composition can be represented by the equation (1), below: EQU Rb=Bp.sup.n (1)
where,
Rb=burning rate (linear) PA1 B=constant PA1 P=pressure PA1 n=pressure exponent, where the pressure exponent is the slope of the plot of the log of pressure along the x-axis versus the log of the burn rate alone the y-axis
Gas generant compositions commonly utilized in the inflation of automotive inflatable restraint airbag cushions have previously most typically employed or been based on sodium azide. Such sodium azide-based compositions, upon initiation, normally produce or form nitrogen gas. While the use of sodium azide and certain other azide-based gas generant materials meets current industry specifications, guidelines and standards, such use may involve or raise potential concerns such as involving the safe and effective handling, supply and disposal of such gas generant materials.
Certain economic and design considerations have also resulted in a need and desire for alternatives to azide-based pyrotechnics and related gas generants. For example, interest in minimizing or at least reducing the overall space requirements for inflatable restraint systems and particularly such requirements related to the inflator component of such systems has stimulated a quest for gas generant materials which provide relatively higher gas yields per unit volume as compared to typical or usual azide-based gas generants. Further, automotive and airbag industry competition has generally lead to a desire for gas generant compositions which satisfy one or more conditions such as being composed of or utilizing less costly ingredients or materials and being amenable to processing via more efficient or less costly gas generant processing techniques.
As a result, the development and use of other suitable gas generant materials has been pursued. In particular, such efforts have been directed to the development of azide-free gas generants for use in such inflator device applications. In view of the above, there is a need and a desire for an azide-free gas generant material that, while overcoming at least some of the potential problems or shortcomings of azide-based gas generants, may also provide relatively high gas yields, such as compared to typical azide-based gas generants. In particular, relatively low cost gas generant material solutions to one or more such problems or limitations are desired.
Through such developmental work, various combinations of fuels and oxidizers have been proposed for use as gas generant materials. Ammonium nitrate is a relatively low cost, commercially available material which, when combined with an appropriate fuel material, may provide or result in relatively high gas output. Unfortunately, certain disadvantages or shortcomings may be associated with the use of ammonium nitrate as the sole oxidizer of such gas generants. For example, such use may result in a gas generant material having a relatively low burning rate, a relatively high burning rate pressure exponent (i.e., the burning rate of the material has a high dependence on pressure) and relatively high hygroscopicity.
In view thereof the burning rates of certain ammonium nitrate-containing compositions have been enhanced variously through the inclusion of one or more selected additives, e.g., a selected high energy fuel ingredient, or by the addition of co-oxidizers such as ammonium and potassium perchlorate. While the inclusion of such high energy fuel ingredients may enhance the burn rate, further increased burn rates are generally desired. In addition, none of such high energy fuel additives are generally effective in significantly reducing the burning rate pressure exponent, as identified above. As will be appreciated, a relatively low burning rate pressure exponent is generally desirable for such compositions such as to reduce the ballistic variability of corresponding airbag inflator devices. In practice, most ammonium nitrate-containing gas generant compositions have a burning rate pressure exponent of approximately 0.75, which is very high relative to the generally desired level of less than 0.60.
Moreover, the inclusion and use of the latter co-oxidizers in gas generant formulations, such as for airbag applications, may be deemed objectionable due to possible concerns regarding toxicity of effluent gas (e.g., formation of objectionable HCl gas) and difficulty in filtering certain undesirable by-products (e.g., alkali metal chlorides) from the gas stream of the associated inflator device.
In addition, ammonium nitrate is known to typically undergo various changes in crystalline structure over the normally expected or anticipated range of storage conditions, e.g., temperatures of about -40.degree. C. to about 110.degree. C. These changes in structure typically involve expansion and contraction of the solid material. Such changes, even when relatively minute, can strongly influence the physical properties of a corresponding gas generant material and, in turn, strongly affect the burn rate of the generant material. Unless checked, such changes in ammonium nitrate structure may result in such performance variations in the gas generant materials incorporating such ammonium nitrate as to render such gas generant materials unacceptable for typical inflatable restraint system applications.
Thus, there is a continuing need and a demand for an azide-free gas generant material that, while overcoming at least some of the potential problems or shortcomings of azide-based gas generants, may also provide relatively high gas yields, such as compared to typical azide-based gas generants, and which provides or results in a sufficientness and desirably high burning rate and low burn a rate pressure exponent.